What is "spacetime" really?

[Michael Mooney]

Pyrotex:

Another viewpoint on Time:
GR tells us that time "runs slower" in a gravitational field. Yet (as we know from other threads) we do not have a good understanding of just what the gravitational "force" is.
....
So, gravitational force derives from the Time Gradient. (?)
But this all depends on what I mean by "energy state".

Prior to the latter is the ontological question, "what do we mean by 'time'?"

I agree with watcher (for a change!) that time is about movement... its "clocked duration" to be specific.
(See my post at the end of the "What is Time" thread and somewhere in this one too.)

It is a leap without a logical imperative to go from the fact that clocks slow down under different conditions relative to each other to "therefore time slows down"... etc.
You, and relativity theorists in general assume the latter without establishing the former imperative or demonstrated link.

Michael

[Essay]

"...its "clocked duration" to be specific." -M.Mooney

The point is not that "clocks" slow down, but reality slows down. Relative to an observer in a spaceship, the clock doesn't slow down. The observer's metabolism and brain processes match the clock and so no "slowing" is observed; but after the spacetrip is over, the observer's clock, metabolism and thoughts all can be observed to have happened over a shorter duration than things that didn't take the same trip.
...or maybe it's just that the rest of reality speeded up during the observer's spacetrip; but whichever....

It is all of reality that endures differently in different gravitational gradients.
It is all of reality that endurates differently in different gravitational gradients.
It is all of reality that endures (has duration) differently in different gravitational gradients.
...one of these three sentences should convey the idea I'm trying to say.

And whether it is the destination (gravi-gradient) or the path taken....
===

Hey[random thought] ! Slowly going to a place with a different gravigradient and staying there for a while and then returning will have the same effect as rapidly travelling to that place and not stopping for the same "while."

Well, depending on if the gravigradient is higher or lower than the origin.
...a higher gravigradient would make that sentence true, but a lower one would....

Okay, now that lower gravigradient would allow for faster ageing, and if you stayed there long enough, it could compensate for the fast trip to that low gravigradient place.

BUT! That place is itelf moving--orbiting some other gravity source....

The speed of the orbits causes some slowing, but the position in the gradient stays the same.
The lower the gradient, the faster the time.

[...just thinking about] Travelling along vs. across the gravigradient--of earth, the sun, the galactic plane, the galactic center, the Great Attractor, the Center? around which the GA orbits, and possibly more "centers."

The lower the gradient, the faster the time.
So out in deep space (with lower gradients), like between the galaxies, wouldn't time run faster.
Wouldn't space "expand" faster?
Or, wouldn't space farther away from an atom expand faster than space near an atom?

[Realizing that at any instant, a given atom's electron might be all the way across the universe--albiet the very low probability]
===

uhhhh.... What did I start talking about...?

Thanks Michael, I'll be back later (I've gotta see a man about a dog--or "dogs" in general)...
...if I can find my way back from this little stream-of-consciousness trip.

[Boof-head]

The ontological/epistemological conundrum seen in how relativistic-domain time is a matter of connections (in an 'imaginary' past and future) changing places - time and space commute in Lorentz transforms - and the 'speed' or motion in our low-energy time domains, compared with when the connections at the end where the absence of temperature, and mass - already absent in optical frequency/time exchanges (if not from the 'outer' environment that these quantum 'spacetime cavities' or condensates, vortical states like the plateau effect in Hall liquids, etc) - where matter looks most like an ordered lattice of some kind, say, time or position, and space or momentum don't commute.

The diffusion gradient is not 'smooth' for Minkowski's lightspace here, the Lorentz connection doesn't find a tangent on any of the inner curves in these lattices.

Let's see if I can rephrase that: in a sense, the tangent space of GR doesn't commute with the tangent space of QM; the geometry of say, a Lorentz frame-shift, will meet an algebra, framed in terms of Boolean operators (on momentum, and orbital modes inside a potential well); we have to construct a parallelized version of a Maxwellian manifold (we align particle spins, or nuclear spin-precession rates), strong magnetic fields are the 'background effect' in spin-polarization circuits.

The largest machine we have ever constructed (in scale, energy-wise, design-wise, most-scientists and PhDs-wise, etc) is designed to 'find' a result, based on the use of a lot of polarization - of charge, to accelerate charged particles, and spin to 'focus' a group of such particles into a beam; the 'real' process occurs at an energy, and at a distance so vanishingly small that spacetime will be almost meaningless - the Planck scale will be approached, we have to use large-scale 'collectors' at much greater distances, to capture the cross-section of the small-scale interactions.

[Michael Mooney]

The point is not that "clocks" slow down, but reality slows down.

Ummmm.... "Reality slows down"??
Say, just for a moment, that you took a flying leap out of the "relativity paradigm" and landed in what most folks *think* is impossible... a cosmic perspective on "Reality.
(Just for a moment... please... you can go back to relative perspective immediately after this little "thought experiment. But *please* "go with it" for this exercise.)

"Seeing" (or even Being!!... a thought experiment, remember!) Cosmos aa a whole... everything everywhere is all happening right now. The speed limit of light only limits what local perspectives can "see," and even the sunlight we see now left the sun over eight minutes ago. So, there is, of course, a time delay in what we can see... the farther away the event/lightsource, the longer the delay.
OK... back to "Reality" with a capital 'R'. Everything everywhere is moving... on all scales from subatomic to universal/cosmic. We all know about these cycles on all scales ('cept maybe the Big Cycle of a possible cyclic cosmos!) Earth spins in "real time*" "one rev a "day" and orbits sun once a "year." Etc., etc... Galaxies spin one rev every so many million "earth years"... etc. (*Real time* in this sense is "absolute* meaning natural cycles that have nothing to do with human (or other intelligent life form) "observation" or the limits of relative perspective.

Are you with me so far??

So... from this "cosmic perspective" what would be the meaning of your phrase, "Reality slows down?" It would lose its meaning outside the paradigm of relative perspective.

So then, what would the concept of gravitational slowing down of time mean? Absolutely nothing! It is just a myopic, relativistic paradigm... as if human perspective and our poor instruments of measurement... (like clocks that slow down under variable gravitational conditions) actually dictate that "Reality slows down!"

Is there anyone here who can see the anthropomorphic absurdity of this?
I think not, and I am sorry about that.

Now, granted, this was presented as a "thought experiment." But then so was the Minkowski/Einstein/relativity *theory* of "spacetime" and in particular "dilated time."

So... the above can be seen as just another thought experiment. It is in fact, the way I have "seen" Reality all my life. ... And It doesn't "slow down" just because our clocks can't keep absolute cosmic time under the stress of various gravitational and other inertial changes "relative to each other."

Just a little food for thought to sleep on.
Hope it doesn't give anyone nightmares!

Michael

Moderation note: this post and 91 responses to it and following posts have been move to thread “Assertion of an ‘absolute now’ from ‘What is “spacetime' really"’” , because they concern the assertion that an absolute “now” exists, while the original thread is excessively long, and a more general discussion of spacetime.

[modest]

Originally Posted by Pyrotex

Another viewpoint on Time:
GR tells us that time "runs slower" in a gravitational field. Yet (as we know from other threads) we do not have a good understanding of just what the gravitational "force" is.

...So, gravitational force derives from the Time Gradient. (?)

I like this question. It would be easy to interpret GR very rigidly and say: yes, the curvature of time is absolutely the reason objects follow an inertial path which accelerates them toward a massive body (i.e. gravitational force). But, I don’t think that interpretation (as valid as it is) needs to be considered the cause of the force.

Changing up the question a bit we could ask how clocks act in a rocket and apply the equivalence principle. If a rocket is accelerating in a straight line far from any gravitational field then a clock on the ceiling (toward the front or nose of the rocket) will run faster than an equivalent clock on the floor. If the clock on the ceiling emits x flashes of light per second then an observer on the floor will receive more than x flashes per second by Doppler shift alone.

The light transit time from the top of the accelerating rocket to the bottom is t=h/c. Change in velocity is acceleration times change in time (v=at). Putting these together gives v=a(h/c). Doppler shift is 1+v/c and subbing the difference in velocity gives 1+ah/c^2. So, the clock on the ceiling runs 1+ah/c^2 faster than an equivalent clock on the floor while the rocket is accelerating. The equation for gravitational time dilation is the same (1+gh/c^2) and so too the equivalence principle says these situations (a rocket accelerating and a person standing on a planet) are equivalent.

So why is a test particle on the ceiling of an accelerating rocket forced to the floor? Is it because of curved time (or alternatively stated: “the time gradient”)? While the inside of the rocket can be described with curved time such as with Rindler coordinates, I don’t think that would usually be said to cause the pseudo (or as they say now: "fictitious") force. More likely, we would say the rocket's engines are responsible or perhaps conservation of momentum or inertia.

I see no reason why we shouldn't look at gravity the same way. Curved time describes the situation that massive bodies present, but I don't think that should necessarily be considered the cause of the pseudo force. The cause would be the thing that makes the freefalling frame the inertial one—the thing that causes time to present as curved. I don't think GR explains what that cause is more than to say it is mass (or more properly: energy, momentum, and stress). But why does mass do that? In other words: what is the ontology of curved time?

Clearly there could be many ontological elements which could be said to be the cause and I'm sure many would be quite valid. I don't think GR constrains the ontology of the force of gravity as much as a lot of people tend to think. The "time gradient" can be considered a description of the pseudo force (in both the cases of the rocket and the planet) but it doesn't explain the cause of the pseudo force.

~modest

[lawcat]

Originally Posted by Rade

moment = that which connects the end of the before (past) with the beginning of the after (future).



Existence is an axiomatic concept, all that can be said about it is that 'existence exists'. All philosophy begins with these two words.

object (thing) = an entity of a specific nature made of specific attributes.

In your philosophy Doctordick an object would be what you term a 'valid ontological entity'

entity = that which exists as distinquished from nothing of non-existence.

===

I do hope these definitions help you better understand my definition of OP question:

'spacetime' = that which is intermediate between two moments of existence.

Beautifully said.

[Doctordick]

Originally Posted by maddog

From what I have seen used when considering coordinate system representation is on a local scale and not global. In fact when considering the radius of curvature is dependent if at all on anything it would be "hidden variable" or parameter. One then uses a parametric representation (say that parameter is the local mass at that coordinate point) to define the curvature. Of course Cosmology does try to apply this on a global scale (with possible pitfalls).

What you are ignoring is the fact that any non-Euclidean geometry possesses fixed relationships between the coordinates in order to establish that geometry.

The only fixed relationship in a Euclidean geometry is the trivial issue of scale of the coordinates which, as a matter of fact, can be totally independent (all one need do is establish that scale in the measure ). Thus Euclidean geometry is the only geometry which can be used to display absolutely any geometric structure. Non-Euclidean geometries inherently limit the possibilities.

Originally Posted by maddog

I don't see what I am doing is "bullying". I can not speak for others. After 750+ some posts to this thread, I have become to think of this more about the discussion between the "representation" of the "thing" vs the "thing" and how that can confuse people. Pointers definitely confuse any newbie programmer first attempt at using them.

I don't really argue with that. I think most all your posts seem quite rational but they are nonetheless not completely thought out.

A comment on "pointers"! They are essentially the information contained in a "goto" statement; and modern programmers always tell me that good programing does not use "goto" statements but every time I go to look at the actual machine language representation of most any modern program I find them salted throughout with long lists of "goto" instructions. Sometimes just long strings of "goto"s pointing to "goto"s. Of course, I am so old that most of my serious "programing" was in machine language; something nobody seems to do anymore.

The central issue of my Ph.D. thesis was "how to squeeze a complex nuclear computation into a machine where the core memory was 64K". That was back when you needed a pickup truck to carry a 300k disk drive and things were so slow that swapping out core was all but inconceivable. I once told my thesis advisor that the smartest approach to our problems would be to go drink beer and party until they built more powerful machines. He didn't appreciate it but, looking back, I think I was right.

Have fun -- Dick

PS before one can talk about space time one needs a good self consistent definition of both "space" and "time"; something totally lacking in this thread.

[lemit]

I've been sitting here listening to traffic outside my house.

Michael, how do you explain the difference in sound as each automobile passes? The space the sound travels through should be the same each direction. The time needed for the sound to arrive should be the same each direction. What is it that happens to the sound?

That's something I've wondered for a long time, since on the farm we could hear a car coming from a long distance and hear it leaving from a shorter distance. The difference in the tone and the duration of the sound wasn't explained in any of our antiquated science books.

We had antiquated schools and antique teachers too, so I was raised on the theory that atoms were the smallest, indivisible units of nature, even as split atoms proliferated around the world. I also had some trouble understanding that plasma rolling around after liightning strikes, too.

So, some childhood questions weren't resolved and I still enjoy thinking about them. I know about Doppler Shift now, but it seems to me that Doppler Shift is a slippery slope to a relativistic understanding of space and time. I'm open to a better explanation. Michael?

--lemit

[Michael Mooney]

Hi lemit,
I must be misunderstanding your question.
Here is the Wiki intro on the speed of sound:

Sound is a vibration that travels through an elastic medium as a wave. The speed of sound describes how much distance such a wave travels in a certain amount of time. In dry air at 20 °C (68 °F), the speed of sound is 343 meters per second (1,125 ft/s). This equates to 1,236 kilometers per hour (768 mph) or about one mile in five seconds.

Then, of course, the Doppler effect makes the sound more high pitched coming at you than going away after it passes. Just more compressed air waves in the former and more stretched out in the latter.
Michael

[lemit]

Thanks, Michael.

I thought I was really onto something, but I guess I wasn't. I really am here to learn, so I like being disabused of my thinking.

I've noticed you seem to treat me with some respect. I appreciate that. I would appreciate your opinions more if you were more respectful of the other people who disagree with you. I think I'm right in saying you were the one who started the name-calling. It would be nice to see you stop it.

Actually, since this is your thread, stopping the name-calling maybe should be your responsibility anyway. That's another thing I'm here to learn: how these forums work. Also, I've never in my life discussed science with scientists. I'm learning how scientists treat non-scientists and how they treat each other. I still have a lot to learn.

Again, thanks for the explanation. I'll keep looking for a spacetime definition in everyday life.

--lemit